A grain-oriented electrical steel sheet has a crystal texture that a bearing of crystal particles is oriented in (110)[001] direction and also has dramatically excellent magnetic characteristics in a pressing direction, so it has been used for an iron core material for a transformer, a motor, a generator, and other electronic devices.
In order to improve an insulation property and to enhance a close contacting property of coating layer, the grain-oriented electrical steel sheet is coated with an insulation coating layer, but recently, the grain-oriented electrical steel sheet is increasingly required to have a low iron loss, so the final insulation coating layer is seek to have a high tension. Actually, the insulation coating layer having a high tension significantly improves the magnetic characteristics of final product, so the various methods for controlling the process factors have been suggested to improve characteristics of the tension coating layer.
As the conventional method of applying a tensile stress to the grain-oriented electrical steel sheet, it is known to use the coefficient difference of thermal expansion between the insulation coating layer formed on the forsterite-based coating layer and the electrical steel sheet, and thereby, the tensile stress is applied to the steel sheet to provide the effects on reducing the iron loss.
The coated electrical steel coil is supplied in a hoop shape, and it is produced in a laminated core transformer and a wound core transformer according to the usage thereof. Particularly, in a case of the wound core for a pole mount transformer, sheet-shaped products are laminated and then cut and formed to be wound with copper and then performed with a stress relief heat treatment for removing stress generated by the process.
In this case, the iron loss may be even deteriorated after the stress annealing according to the heat treatment condition, causing the efficiency deterioration of transformer.
Particularly, in order to ensure the stress relieving effects within a short time, it is usually heat-treated at a high temperature of greater than or equal to 850° C., but, in this case, almost products occur iron loss deterioration, resulting that the efficiency of transformer is deteriorated.
Thus, grain-oriented electrical steel sheet product is required not to deteriorate iron loss even after the stress relief annealing at a temperature of greater than or equal to 850° C., and also to have excellent insulation.